Absolute position encoders use a scale structure that allows the position of a readhead relative to a scale to be determined without requiring any reference to a given starting point along the scale. A variety of absolute position encoders are known which use multiple information tracks along the scale.
In many conventional capacitive and inductive absolute position encoders, the scale structures include two or more tracks that have different periodic spatial wavelengths that, in combination, uniquely identify the position of the readhead relative to the scale within a limited distance. U.S. Pat. No. 5,886,519 discloses one embodiment of such an inductive absolute position encoder.
The '519 patent also discloses an embodiment that includes a track having a periodic spatial layout of the sensible elements and a separate, code, track that includes a series of sensible elements that are arranged in a pseudo-random sequence. In particular, for any subset of the sensible elements of this code track that are located within the bounds of the readhead sensor elements, the sensible elements form a unique binary, trinary or higher-modulus code word that uniquely identifies the position of the readhead relative to the scale to a first resolution, while the track having a periodic spatial layout identifies the position of the readhead relative to the scale to a finer resolution.
Similarly, many conventional optical position encoders use multiple information tracks along the scale, where, for each “position” along the scale, a code track, or a combination of code tracks, defines a unique value that uniquely identifies that position along the scale. Often, methods are also used which determine the position of a periodic track relative to the readhead to a resolution that is smaller than the resolution provided by the one or more code tracks alone. U.S. Pat. Nos. 3,882,482, 5,965,879 and 5,279,044, incorporated herein by reference, disclose such multi-code-track optical encoders.
U.S. Pat. No. 4,631,519, as well as the '044 patent, disclose embodiments of optical absolute position encoders that integrate the previously mentioned code track functions and periodic track functions into parallel tracks or a single combined track. In such embodiments, the sensible code elements are arranged in a pseudo-random sequence along a track, with each individual sensible code element positioned to align with a continuous periodic pitch of an incremental track such that a displacement of a pick-up device over one pitch of the incremental track always produces a specific code for each pitch.